I played it once, but I didn't really like playing with other people because most of them were stupid and inarticulate. I pretty much quested my way to 80, and then quit. It was mostly a test of my endurance, rather than because I enjoyed the game.
I did make one or two friends, one of whom would talk to me all the time, and rescued me a couple of times when I got into trouble. I was an undead mage named Altea on Blackhand. It was a very funny dynamic... I was running around relying on arcane magic to blast my enemies, and largely managing solo. Whenever I found myself in a tight spot, my Death Knight friend would rescue me. He always insisted on running me through instances, which usually got me armor/weapons that were better than anything I'd find normally for the next 20 levels or so.
Let's just say that playing the game that way, I ended up having to think up some interesting strategies in Outlands in order to compensate for my lack of a group. That was probably the hardest part of the game.
My fondest memory of the game is probably when I was questing in Tanaris Desert and had to kill several enemies. I ended up playing the Gerudo Valley music while I was there in order to indulge in some OoT nostalgia. Another favorite is riding around the world on horseback and into Alliance areas to get the World Explorer achievement right before I quit. It was very freeing to just wander through the areas with no worries and no goals.